Intake systems disclosed in, for example, JP 2001-165012 A and JP 2000-145594 A, for an internal combustion engine, are equipped with an intake silencer forming an expansion chamber to reduce intake noise. An intake system, for an internal combustion engine, disclosed in, for example, JP 2-49963 A is provided with an intake air silencing device provided with a resonance chamber in addition to an intake silencer.
When intake noise in a specific frequency range cannot be sufficiently effectively damped only by the intake silencer forming an expansion chamber, namely, expansion silencer, an intake silencer forming a resonance chamber, namely, resonance silencer, is used in combination with the expansion silencer. The intake silencer is large if the expansion silencer and the resonance silencer are installed separately. Therefore, it is difficult to compactly incorporate such a large intake silencer into a machine formed in extremely compact construction, such as an outboard motor.
An internal combustion engine for an outboard motor is disclosed in, for example, JP 10-184469 A. This known internal combustion engine has an intake silencer forming an expansion chamber and connected to a throttle device by a short, flexible connecting pipe. A known internal combustion engine disclosed in, for example, JP 2000-145594 A has an intake silencer connected to a throttle device connected to a cylinder head by an intake pipe extending in the intake silencer.
When an intake silencer and a throttle device are connected by an elastic connecting pipe having rubber elasticity, vibrations of an engine body are not readily transmitted to the intake silencer and, consequently, noise resulting from the vibration of the intake silencer is reduced. On the other hand, since the intake pipe connecting the intake silencer and the throttle device extends in the intake silencer, engine output can be increased by an intake inertia effect, and intake noise in a specific frequency range can be reduced by adjusting the length of part of the intake pipe extending in the intake silencer without entailing increase in the size of the intake system which will result from increase in the interval between the intake silencer and the throttle device. However, the intake silencer and the throttle device cannot be connected by a long elastic connecting pipe having rubber elasticity because the connecting pipe is flexible.